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From Aljoscha Krettek <aljos...@apache.org>
Subject Re: [DISCUSS] FLIP-17 Side Inputs
Date Fri, 17 Mar 2017 15:03:10 GMT
Yes, I agree! The implementation stuff we talked about so far is only
visible at the operator level. A user function that uses the (future)
side API would not be aware of whether buffering or blocking is used. It
would simply know that it is invoked and that side input is ready.

I'll also quickly try to elaborate on my comment about why I think
windowing/triggering in the side-input operator itself is not necessary.
I created a figure: http://imgur.com/a/aAlw7 It is enough for the
side-input operator simply to consider side input for a given window as
ready when we have seen some data for that window. The WindowOperator
that is upstream of the side input will take care of
windowing/triggering.

I'll create Jira issues for implementing the low-level requirements for
side inputs (n-ary operator, broadcast state and buffering) and update
this thread. If anyone is interested on working on one of those we might
have a chance of getting this ready for Flink 1.3. Time is a bit tight
for me because I'm going to be on vacation for 1.5 weeks starting next
week Wednesday and after that we have Flink Forward.

Best,
Aljoscha

On Thu, Mar 16, 2017, at 23:52, Gábor Hermann wrote:
> Regarding the CoFlatMap workaround,
> - For keyed streams, do you suggest that having a per-key buffer stored 
> as keyed state would have a large memory overhead? That must be true, 
> although a workaround could be partitioning the data and using a 
> non-keyed stream. Of course that seems hacky, as we have a keyed stream 
> abstraction, so I agree with you.
> - I agree that keeping a broadcast side-input in the operator state is 
> not optimal. That's a good point I have not thought about. First we have 
> a separate abstraction for broadcast state, then we can optimize e.g. 
> checkpointing it (avoiding checkpointing it at every operator).
> 
> 
> Regarding blocking/backpressuring inputs, it should not only be useful 
> for static side-input, but also for periodically updated (i.e. slowly 
> changing). E.g. when a machine learning model is updated and loaded 
> every hour, it make sense to prioritize loading the model on the side 
> input. But I see the limitations of the underlying runtime.
> 
> Exposing a buffer could be useful for now. Although, the *API* for 
> blocking could even be implemented by simply buffering. So the buffering 
> could be hidden from the user, and later maybe optimized to not only 
> buffer, but also apply backpressure. What do you think? Again, for the 
> prototype, exposing the buffer should be fine IMHO. API and 
> implementation for blocking inputs could be a separate issue, but let's 
> not forget about it.
> 
> Cheers,
> Gabor
> 
> 
> On 2017-03-15 16:14, Aljoscha Krettek wrote:
> > Hi,
> > thanks for you input! :-)
> >
> > Regarding 1)
> > I don't see the benefit of integrating windowing into the side-input
> > logic. Windowing can happen upstream and whenever that emits new data
> > then operator will notice because there is new input. Having windowing
> > inside the side-input of an operator as well would just make the
> > implementation more complex without adding benefit, IMHO.
> >
> > Regarding 2)
> > That's a very good observation! I think we are fine, though, because
> > checkpoint barriers never "overtake" elements. It's only elements that
> > can overtake checkpoint barriers. If the broadcast state on different
> > parallel instances differs in a checkpoint then it only differs because
> > some parallel instances have reflected changes in their state from
> > elements that they shouldn't have "seen" yet in the exactly-once mode.
> > If we pick the state of an arbitrary instance as the de-facto state we
> > don't break guarantees any more than turning on at-least-once mode does.
> >
> > Regarding 3)
> > We need the special buffer support for keyed operations because there we
> > need to make sure that data is restored on the correct operator that is
> > responsible for the key of the data while also allowing us to iterate
> > over all the buffered data (for when we are ready to process the data).
> > This iteration over elements is not possible when simply storing data in
> > keyed state.
> >
> > What do you think?
> >
> > On Wed, Mar 15, 2017, at 09:07, wenlong.lwl wrote:
> >> Hi, Aljoscha, I just go through your prototype. I like the design of the
> >> SideInputReader which can make it flexible to determine when we can get
> >> the
> >> side input.
> >>
> >> I agree that side inputs are API sugar on the top of the three
> >> components(n-ary
> >> inputs, broadcast state and input buffering), following is some more
> >> thought about the three component:
> >>
> >> 1. Take both N-ary input operator and windowing/triggers mechanism into
> >> consideration, I think we may need the N-ary input operator supports some
> >> inputs(side inputs) are windowed while the others(main input) are normal
> >> stream. for static/slow-evolving data, we need to use global windows and
> >> for windowed-base join data , we need to use time window or custom
> >> windows.
> >> The window function on the side input can be used to collect or merge the
> >> data to generate the value of the side input(a single value or
> >> list/map).
> >> Once a side input reader window is triggered, the SideInputReader will
> >> return value available, and if a Window is triggered more than once, the
> >> value of side input will be updated and maybe the SideInputReader need a
> >> interface to notice the user that something changed. Besides, I prefer
> >> the
> >> option to make every input of N-ary input operator equal, because user
> >> may
> >> need one side input depends on another side input.
> >>
> >> 2. Regarding broadcast state, my concern is that how can we merge the
> >> value
> >> of the state from different subtasks. If the job running in at least once
> >> mode, the returned value of broadcast state from different subtasks will
> >> be
> >> different. Is there already any design on broadcast state?
> >>
> >> 3. Regarding input buffering, I think if we use window/trigger mechanism,
> >> state can be store in the state of window, which may be mostly like what
> >> we
> >> need to do currently in KeyedWindow and AllWindow. We may need to allow
> >> custom merge strategy on all window state data since in side inputs we
> >> may
> >> need to choose data according to broadcast state strategy  while in
> >> normal
> >> windows we can just redistribute the window state data.
> >>
> >> What do you think?
> >>
> >> Best Regards!
> >>
> >> Wenlong
> >>
> >> On 14 March 2017 at 01:41, Aljoscha Krettek <aljoscha@apache.org> wrote:
> >>
> >>> Ha! this is turning out to be quite the discussion. :-) Also, thanks
> >>> Kenn, for chiming in with the Beam perspective!
> >>>
> >>> I'll try and address some stuff.
> >>>
> >>> It seems we have some consensus on using N-ary operator to implement
> >>> side inputs. I see two ways forward there:
> >>>   - Have a "pure" N-ary operator that has zero inputs by default and all
> >>>   N inputs are equal: this exists side-by-side with the current one-input
> >>>   operator and two-input operator.
> >>>   - Extends the existing operators with more inputs: the main input(s)
> >>>   would be considered different from the N other inputs, internally. With
> >>>   this, we would not have to rewrite existing operators and could simply
> >>>   have side inputs as an add-on.
> >>>
> >>> There weren't any (many?) comments on using broadcast state for side
> >>> inputs. I think there is not much to agree on there because it seems
> >>> pretty straightforward to me that we need this.
> >>>
> >>> About buffering: I think we need this as a Flink service because it is
> >>> right now not (easily) possible to buffer keyed input. For keyed input
> >>> we need to checkpoint the input buffers with the key-grouped state.
> >>> Otherwise the data would not be distributed to the correct operator when
> >>> restoring. This is explained in the FLIP in more detail.
> >>>
> >>> If we have these three components (n-ary inputs, broadcast state and
> >>> input buffering) then side inputs are mostly API sugar on top. I even
> >>> believe that it might be enough to simply provide these and then users
> >>> have a very flexible system that allows them to implement different
> >>> side-input variants. I'm suggesting this because I see there are a lot
> >>> of different opinions and because the "field" of determining a side
> >>> input to be finished is still quite open.
> >>>
> >>> Now, regarding Gabor's comments which, I think, pretty nicely summed up
> >>> the ongoing discussion and added some new stuff:
> >>>
> >>> About the CoFlatMap for the simple case: I think this is almost
> >>> possible, except for the buffering in case of a keyed input stream.
> >>> Also, the side input is not easy to store because we need broadcast
> >>> state for that (depending, of course, on whether the input(s) are keyed
> >>> or not). I think with the above-mentioned additions this case would be
> >>> possible without explicit support for side inputs in the API.
> >>>
> >>> Re 1)
> >>> I would prefer to use windowing/triggers for determining side-input
> >>> readiness. There are, right now, enough messages flying around the
> >>> system and introducing yet more doesn't seem to desirable for me right
> >>> now. We should, of course, revisit this once we have the basic
> >>> components in place.
> >>>
> >>> Re 2)
> >>> See my comments about buffering in a keyed operator above. Regarding
> >>> blocking, this is currently not possible because all inputs are consumed
> >>> by one thread. This could, of course, change in the future but it is a
> >>> feature (limitation?) of the current implementation. In general, I think
> >>> blocking an input is only ever feasible while waiting for some bounded
> >>> inputs to be fully consumed. I.e. when you have some initial loading of
> >>> data from a static data set.
> >>>
> >>> Re 3)
> >>> Agreed, I think that we should keep the side-input in the (yet to be
> >>> introduced) broadcast state. Again, once we have the basics in place we
> >>> can investigate further optimisations here such as not checkpointing
> >>> side-input data from a static data set because we know that we can
> >>> easily rebuild it.
> >>>
> >>> What do you think?
> >>>
> >>> On Fri, Mar 10, 2017, at 20:44, Kenneth Knowles wrote:
> >>>> Hi all,
> >>>>
> >>>> I thought I would briefly join this thread to mention some side input
> >>>> lessons from Apache Beam. My knowledge of Flink is not deep enough,
> >>>> technically or philosophically, to make any specific recommendations.
And
> >>>> I
> >>>> might just be repeating things that the docs and threads cover, but
I
> >>>> hope
> >>>> it might be helpful anyhow.
> >>>>
> >>>> Side Input Visibility / matching: Beam started with a coupling between
> >>>> the
> >>>> windowing on a stream and the way that windows are mapped between main
> >>>> input and side input. This is actually not needed and we'll be making
the
> >>>> mapping explicit (with sensible defaults). In particular, the mapping
> >>>> determines when you can garbage collect, when you know that no main
input
> >>>> element will ever map to a particular window again (so opaque mappings
> >>>> need
> >>>> some metadata).
> >>>>
> >>>> Side Input Readiness: There is an unpleasant asymmetry between waiting
> >>>> for
> >>>> the first triggering of a side input but not waiting for any later
> >>>> triggering. This manifests strongly when a user actually wants to know
> >>>> something about the relationship to side input update latency and main
> >>>> input processing. This echoes some of the concern here about user-defined
> >>>> control over readiness. IMO this is a rather open area.
> >>>>
> >>>> Default values for singleton side inputs: A special case of side input
> >>>> readiness that is related also to windowing. By far the most useful
> >>>> singleton side input is the result of a global reduction with an
> >>>> associative&commutative operator. A lot of these operators also
have an
> >>>> identity element. It is nice for this identity element (known a priori)
> >>>> to
> >>>> be "always available" on the side input, for every window, if it is
> >>>> expected to be something that is continually updated. But if the
> >>>> configuration is such that it is a one-time triggering of bounded data,
> >>>> that behavior is not right. Related, after some amount of time we
> >>>> conclude
> >>>> that no input will ever be received for a window, and the side input
> >>>> becomes ready.
> >>>>
> >>>> Map Side Inputs with triggers: When new data arrives for a key in Beam,
> >>>> there's no way to know which value should "win", so you basically just
> >>>> can't use map side inputs with triggers.
> >>>>
> >>>> These are just some quick thoughts at a very high level.
> >>>>
> >>>> Kenn
> >>>>
> >>>> On Thu, Mar 9, 2017 at 7:59 AM, Aljoscha Krettek <aljoscha@apache.org>
> >>>> wrote:
> >>>>
> >>>>> Hi Jamie,
> >>>>> actually the approach where the .withSideInput() comes before the
user
> >>>>> function is only required for implementation proposal #1, which
I like
> >>>>> the least. For the other two it can be after the user function,
which
> >>> is
> >>>>> also what I prefer.
> >>>>>
> >>>>> Regarding semantics: yes, we simply wait for anything to be available.
> >>>>> For GlobalWindows, i.e. side inputs on a normal function where we
> >>> simply
> >>>>> don't have windows, this means that we wait for anything. For the
> >>>>> windowed case, which I'm proposing as a second step we will wait
for
> >>>>> side input in a window to be available that matches the main-input
> >>>>> window. For the keyed case we wait for something on the same key
to be
> >>>>> available, for the broadcast case we wait for anything.
> >>>>>
> >>>>> Best,
> >>>>> Aljoscha
> >>>>>
> >>>>> On Thu, Mar 9, 2017, at 16:55, Jamie Grier wrote:
> >>>>>> Hi, I think the proposal looks good.  The only thing I wasn't
clear
> >>> on
> >>>>>> was
> >>>>>> which API is actually being proposed.  The one where .withSideInput()
> >>>>>> comes
> >>>>>> before the user function or after.  I would definitely prefer
it come
> >>>>>> after
> >>>>>> since that's the normal pattern in the Flink API.  I understood
that
> >>>>>> makes
> >>>>>> the implementation different (maybe harder) but I think it helps
> >>> keep the
> >>>>>> API uniform which is really good.
> >>>>>>
> >>>>>> Overall I think the API looks good and yes there are some tricky
> >>>>>> semantics
> >>>>>> here but in general if, when processing keyed main streams,
we always
> >>>>>> wait
> >>>>>> until there is a side-input available for that key we're off
to a
> >>> great
> >>>>>> start and I think that was what you're suggesting in the design
doc.
> >>>>>>
> >>>>>> -Jamie
> >>>>>>
> >>>>>>
> >>>>>> On Thu, Mar 9, 2017 at 7:27 AM, Aljoscha Krettek <
> >>> aljoscha@apache.org>
> >>>>>> wrote:
> >>>>>>
> >>>>>>> Hi,
> >>>>>>> these are all valuable suggestions and I think that we should
> >>> implement
> >>>>>>> them when the time is right. However, I would like to first
get a
> >>>>>>> minimal viable version of this feature into Flink and then
expand
> >>> on
> >>>>> it.
> >>>>>>> I think the last few tries of tackling this problem fizzled
out
> >>> because
> >>>>>>> we got to deep into discussing special semantics and features.
I
> >>> think
> >>>>>>> the most important thing to agree on right now is the basic
API
> >>> and the
> >>>>>>> implementation plan. What do you think about that?
> >>>>>>>
> >>>>>>> Regarding your suggestions, I have in fact a branch [1]
from May
> >>> 2016
> >>>>>>> where I implemented a prototype implementation. This has
an n-ary
> >>>>>>> operator and inputs can be either bounded or unbounded and
the
> >>>>>>> implementation actually waits for all bounded inputs to
finish
> >>> before
> >>>>>>> starting to process the unbounded inputs.
> >>>>>>>
> >>>>>>> In general, I think blocking on an input is only possible
while
> >>> you're
> >>>>>>> waiting for a bounded input to finish. If all inputs are
unbounded
> >>> you
> >>>>>>> cannot block because you might run into deadlocks (in the
> >>> processing
> >>>>>>> graph, due to back pressure) and also because blocking will
also
> >>> block
> >>>>>>> elements that might have a lower timestamp and might fall
into a
> >>>>>>> different window which is already ready for processing.
> >>>>>>>
> >>>>>>> Best,
> >>>>>>> Aljoscha
> >>>>>>>
> >>>>>>> [1]
> >>>>>>> https://github.com/aljoscha/flink/commits/operator-ng-side-
> >>>>> input-wrapper
> >>>>>>> On Tue, Mar 7, 2017, at 14:39, wenlong.lwl wrote:
> >>>>>>>> Hi Aljoscha, thank you for the proposal, it is great
to hear
> >>> about
> >>>>> the
> >>>>>>>> progress in side input.
> >>>>>>>>
> >>>>>>>> Following is my point of view:
> >>>>>>>> 1. I think there may be an option to block the processing
of the
> >>> main
> >>>>>>>> input
> >>>>>>>> instead of buffer the data in state because in production,
the
> >>>>> through
> >>>>>>>> put
> >>>>>>>> of the main input is usually much larger, and buffering
the data
> >>>>> before
> >>>>>>>> the
> >>>>>>>> side input may slow down the preparing of side input
since the
> >>> i-o
> >>>>> and
> >>>>>>>> computing resources are always limited.
> >>>>>>>> 2. another issue may need to be disscussed: how can
we do
> >>>>> checkpointing
> >>>>>>>> with side input, because static side input may finish
soon once
> >>>>> started
> >>>>>>>> which will stop the checkpointing.
> >>>>>>>> 3. I agree with Gyula that user should be able to determines
> >>> when a
> >>>>> side
> >>>>>>>> input is ready? Maybe we can do it one step further:
whether
> >>> users
> >>>>> can
> >>>>>>>> determine a operator with multiple inputs to process
which input
> >>> each
> >>>>>>>> time
> >>>>>>>> or not?  It would be more flexible.
> >>>>>>>>
> >>>>>>>>
> >>>>>>>> Best Regards!
> >>>>>>>> Wenlong
> >>>>>>>>
> >>>>>>>> On 7 March 2017 at 18:39, Ventura Del Monte <
> >>>>> venturadelmonte@gmail.com>
> >>>>>>>> wrote:
> >>>>>>>>
> >>>>>>>>> Hi Aljoscha,
> >>>>>>>>>
> >>>>>>>>> Thank you for the proposal and for bringing up again
this
> >>>>> discussion.
> >>>>>>>>> Regarding the implementation aspect,I would say
the first way
> >>> could
> >>>>>>>>> be easier/faster to implement but it could add some
overhead
> >>> when
> >>>>>>>>> dealing with multiple side inputs through the current
2-streams
> >>>>> union
> >>>>>>>>> transform. I tried the second option myself as it
has less
> >>> overhead
> >>>>>>>>> but then the outcome was something close to a N-ary
operator
> >>>>> consuming
> >>>>>>>>> first each side input while buffering the main one.
> >>>>>>>>> Therefore, I would choose the third option as it
is more
> >>> generic
> >>>>>>>>> and might help also in other scenarios, although
its
> >>> implementation
> >>>>>>>>> requires more effort.
> >>>>>>>>> I also agree with Gyula, I think the user should
be allowed to
> >>>>> define
> >>>>>>> the
> >>>>>>>>> condition that determines when a side input is ready,
e.g.,
> >>> load
> >>>>> the
> >>>>>>> side
> >>>>>>>>> input first, incrementally update the side input.
> >>>>>>>>>
> >>>>>>>>> Best,
> >>>>>>>>> Ventura
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>> This message, for the D. Lgs n. 196/2003 (Privacy
Code), may
> >>>>> contain
> >>>>>>>>> confidential and/or privileged information. If you
are not the
> >>>>>>> addressee or
> >>>>>>>>> authorized to receive this for the addressee, you
must not use,
> >>>>> copy,
> >>>>>>>>> disclose or take any action based on this message
or any
> >>>>> information
> >>>>>>>>> herein. If you have received this message in error,
please
> >>> advise
> >>>>> the
> >>>>>>>>> sender immediately by reply e-mail and delete this
message.
> >>> Thank
> >>>>> you
> >>>>>>> for
> >>>>>>>>> your cooperation.
> >>>>>>>>>
> >>>>>>>>> On Mon, Mar 6, 2017 at 3:50 PM, Gyula Fóra <
> >>> gyula.fora@gmail.com>
> >>>>>>> wrote:
> >>>>>>>>>> Hi Aljoscha,
> >>>>>>>>>>
> >>>>>>>>>> Thank you for the nice proposal!
> >>>>>>>>>>
> >>>>>>>>>> I think it would make sense to allow user's
to affect the
> >>>>> readiness
> >>>>>>> of
> >>>>>>>>> the
> >>>>>>>>>> side input. I think making it ready when the
first element
> >>>>> arrives is
> >>>>>>>>> only
> >>>>>>>>>> slightly better then making it always ready
from usability
> >>>>>>> perspective.
> >>>>>>>>> For
> >>>>>>>>>> instance if I am joining against a static data
set I want to
> >>> wait
> >>>>>>> for the
> >>>>>>>>>> whole set before making it ready. This could
be exposed as a
> >>> user
> >>>>>>> defined
> >>>>>>>>>> condition that could also recognize bounded
inputs maybe.
> >>>>>>>>>>
> >>>>>>>>>> Maybe we could also add an aggregating (merging)
side input
> >>> type,
> >>>>>>> that
> >>>>>>>>>> could work as a broadcast state.
> >>>>>>>>>>
> >>>>>>>>>> What do you think?
> >>>>>>>>>>
> >>>>>>>>>> Gyula
> >>>>>>>>>>
> >>>>>>>>>> Aljoscha Krettek <aljoscha@apache.org>
ezt írta (időpont:
> >>> 2017.
> >>>>>>> márc.
> >>>>>>>>> 6.,
> >>>>>>>>>> H, 15:18):
> >>>>>>>>>>
> >>>>>>>>>>> Hi Folks,
> >>>>>>>>>>>
> >>>>>>>>>>> I would like to finally agree on a plan
for implementing
> >>> side
> >>>>>>> inputs in
> >>>>>>>>>>> Flink. There has already been an attempt
to come to
> >>> consensus
> >>>>> [1],
> >>>>>>>>> which
> >>>>>>>>>>> resulted in two design documents. I tried
to consolidate
> >>> those
> >>>>> two
> >>>>>>> and
> >>>>>>>>>>> also added a section about implementation
plans. This is
> >>> the
> >>>>>>> resulting
> >>>>>>>>>>> FLIP:
> >>>>>>>>>>>
> >>>>>>>>>>> https://cwiki.apache.org/confluence/display/FLINK/FLIP-
> >>>>>>>>>> 17+Side+Inputs+for+DataStream+API
> >>>>>>>>>>>
> >>>>>>>>>>> In terms of semantics I tried to go with
the minimal viable
> >>>>>>> solution.
> >>>>>>>>>>> The part that needs discussing is how we
want to implement
> >>>>> this. I
> >>>>>>>>>>> outlined three possible implementation plans
in the FLIP
> >>> but
> >>>>> what
> >>>>>>> it
> >>>>>>>>>>> boils down to is that we need to introduce
some way of
> >>> getting
> >>>>>>> several
> >>>>>>>>>>> inputs into an operator/task.
> >>>>>>>>>>>
> >>>>>>>>>>>
> >>>>>>>>>>> Please have a look at the doc and let us
know what you
> >>> think.
> >>>>>>>>>>>
> >>>>>>>>>>>
> >>>>>>>>>>> Best,
> >>>>>>>>>>>
> >>>>>>>>>>> Aljoscha
> >>>>>>>>>>>
> >>>>>>>>>>>
> >>>>>>>>>>>
> >>>>>>>>>>> [1]
> >>>>>>>>>>> https://lists.apache.org/thread.html/
> >>>>>>> 797df0ba066151b77c7951fd7d603a
> >>>>>>>>>> 8afd7023920d0607a0c6337db3@1462181294@%3Cdev.flink.apache.
> >>> org%3E
> >>>>>>
> >>>>>>
> >>>>>> --
> >>>>>>
> >>>>>> Jamie Grier
> >>>>>> data Artisans, Director of Applications Engineering
> >>>>>> @jamiegrier <https://twitter.com/jamiegrier>
> >>>>>> jamie@data-artisans.com
> 

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